1. Field of the Invention
The present invention relates to a local communication system comprising a plurality of stations interconnected for the communication of messages via a serial data channel. The invention further relates to apparatuses for use in such a system, and diagnostics apparatus for use with such a system.
2. Related Art
A system of this type has been described for example by Knapp and Hetzel in a paper "Audio Local Area Network Chip For Cars", presented at the 92nd Convention of the Audio Engineering Society, Mar. 24-27, 1992, Vienna. Knapp and Hetzel propose use of the well known SPDIF format for the transmission of digital audio signals among the various components of a car audio system, and incorporate within that format eight channels for the transmission of control messages in parallel with the audio signals.
In the system of Knapp and Hetzel, known as A-LAN for short, each station (each apparatus) is constructed and configured to transmit on a respective one of the eight control channels, while receiving messages from others of the stations on the remaining seven channels.
Some practical problems arise in the implementation of such a system. Firstly, the allocation of channels as described limits the number of stations connected in the ring network. The reference indicates how one might exceed this limit by providing multiple rings, with one of the stations acting as a gateway to the other ring. Such a gateway is complicated and costly to implement, however, and effectively increases the cost of adding a ninth station to an A-LAN system.
Secondly, each station may find itself obliged to receive unrelated messages simultaneously on several channels. This necessitates costly communication management provisions, for example to buffer and prioritise these messages. At the same time, limited processing power is available for control messages in a low-cost system, and it is anticipated that often a particular message will not be accepted because the input buffer of the receiving station is still occupied by an earlier message.